Repeaters can be interposed in communication between a mobile station and a base station, in order to overcome attenuation which occurs when the mobile station is located in such a position that a signal transmitted from any base station cannot be properly received. Such a repeater receives a signal from a base station and relays the received signal to the mobile station. Normally, the repeater amplifies the received signal, and outputs the signal to the mobile station at the amplified level.
It is necessary to achieve synchronization between the base station and the repeater, and between the repeater and the mobile station, in order for the communication between the base station and the repeater and between the repeater and the mobile station to be performed.
Between the base station and the mobile station, the communication is performed as shown in FIG. 8. In FIG. 8, Tx indicates a transmission phase, and Rx indicates a reception phase. It is assumed that the repeater is interposed between the base station and the mobile station, so that the signal transmitted from the base station is relayed by the repeater.
Regarding such a communication where the repeater is interposed between the base station and the mobile station, so that the communication between the base station and the mobile station is relayed by the repeater, the WiMAX standards describe the following frame structure to mitigate interference and improve quality in the communication. That is to say, as shown in FIG. 9, the start (i.e. time points T0 and T2 in FIG. 9) of each frame transmitted/received in a path between an upstream end (i.e. the base station) and a downstream end (i.e. the mobile station) is determined so that timing of downlink (i.e. DL in FIG. 9) frames are aligned with each other, and timing of uplink (i.e. UL in FIG. 9) frames are aligned with each other. Such alignment of start timing of each frame is performed across a plurality of communication devices, and is called achievement of synchronization. Note that each frame consists of an UL sub-frame, a DL sub-frame, and a preamble attached before the DL sub-frame for the synchronization purpose, and is configured to be 5 msec long.
As a method for achieving synchronization, a technique is disclosed in Patent Literature 1. According to the technique disclosed in Patent Literature 1, each of the base station, the repeater, and the mobile station is equipped with a GPS (Global Positioning System), and operates in accordance with clock signals which are extremely accurate time information output from the GPS. Synchronization is achieved by adjusting the start of each of predetermined numbers of frames (i.e. 200 frames) so that each frame starts at a rising edge of the respective clock signals (1 clk/sec).
Further, Non-Patent Literature 1 describes the achievement of synchronization according to the WiMAX standards.